Condenser and bushing cap



Juiy 22, 1941.

A. A. EKVORTZOFF CONDENSER AND BUSHING CAP Filed Oct. 28, 1958 L- l VIII IIII'IIIIIMIII.

INVENTOR fl/exanderfl Skmnzaff WITNESSES:

Patented July 22, 1941 UNITED STATES iATENT ()FFI'CE 2,249,863 deems AND BUSHING GAP Alexander A. Skvortzofi, Sharon; Pa.', assignor to Westinghouse Electric & Manufacturing Company, East littsburgh, Pa., a corporation of Pennsylvania maniacal o'cteiier 28,1938, Serial N6. 237,483

6 Claims.

Th piean invention relates to insulating iijshing s of the type used to insulate the leads of h g -tension electrical apica'ratus such as transreinii aiifd circ'iiit br aker's', and morep'articulariy to a bushing construction in which means 1s prov a to jacilitate the testing of the insulatmg v "e' of the bushing independentl of the lead eittend g thrbugh the b'ushing an the apiarati scennetei tb such lead. In bushings of t ence eentm iateat er is usua i provided a p elaiii shell or Weather casing through which ext aco'rioiucting lead from a transformer or circuit Breaker encased in a tank upon which the Bushing is mounted. One end of the p'orce lain sheli is p o'viaed with a tern'iina'l cap to which the ccnducting iead is secure and which facilitates the connection of the bushing l'agi to an e'iits'id circuit. The conducting lead which may be t'baiar' or solidis usually surrounded by suitable insulation such as porcelain or fibrous canteen, or a wrapping of impregnated or cp'atee patie r or fabric, or a Wrapping of the condenser type, and the snen isfilld around the insulation with a suitable insulating fluid such as 011 or gum.

It is now gener lly accepted that bushings Of the time cor'iteh platd gnome be periodically tested for vlosses in' the insulation, and it is also dsijra e" to tstth'em, of course, after assembly being put into use After the bushing f use, a; is desirable that it be tested in? i bisit "n n the f el without di s assembling it f1 the ,ehcaseiijapparatus on which it is mjou ed. The insulation loss test usual-1y is'n'xla'de by he vtattnietermetlfod as disclosed in pebie Patnt' N0 1, 9}i5,263', or by a bi method iving qire' t readings roportional d, power iactor as disclosed in w en a1. Patna No. 2,130,865 issued September 20, 1938.

In tests of this character, pafe riaiy in fieid tsting after biishing is se ryice a test of the bush'ng the us' u'ai n annenby applying the to, ca citance a ts tpotmiarm the teifniinai of the bush g,- n pi'ess the test pbtential not on1y between the thisniannr, it apiiet' be uetermiiifd 2 I wher' theioss may hegand it is ns ia l iy nee sary to disassemble the bushing lead from the appa ratus in the tanic, ordrop theleadirom the bushing into the tank andtest the various elements separately to determine whichis the defective part or where the undesirable loss is occurring.

This, of course, involves a substantial expenditure o f time with a corresponding expense and. in View of the time and expense requireci, the test is often neglected with theresult that the appa r atus n iay operate for some time in a dangerous condition before the danger is noticed or the apparatus iails'. r I V T In copending application Seria1. No. 237,284, filed Oct ber Z'Y, 1938 hy me and. Fred voge1, I have disclosed one means for remedying the alccve condition and the present invention is a modification to take care of bushings haying solid or tubular studs or leads. In accordance with the present invention an auxiliary conducting cylinder surrounds the bushin lead, or stud, in insulated relation andconnection may be made toit iron; the test circuit to test the insulation. and thereafter the auxiliary cylinder and. stud ma be brought tothe sarne electrical pqilential for normal operation or the b'ushing if desired.

It is an object oi the present invention to provide a bushing construction for insulating the leads oi: electrical apparatus which permits measurement of the bushing losses independent of any losses which maybe attributable to defects inthe apparatus with which the bushing is assoa It is a further objectof the invention to provi'd'esuch a bushing construction whereby such measurement may be made even though the lead or stud of the bushing is continuously c nected to the terminal of the bushing, and not insulated therefrom. i

1 Other objects will be apparent from the following description when read in connection with the accompanying drawing in which I v v Figure 1 is a'view in elevation with parts broken away of a bushing assembled in accordance with the present invention; accompanied by eschematic diagram of a test circuit for impressing a test potential onthe bushinginsulation; and

Fig. 2 is a vievii partly in vertical section, with parts broken away, of a portion of the structure show'n'in Fig 1. h v

Referring more specifically to, Fig. 1, pheassexnbleti bushing, as'is u'sual vpractice, comprises 'a cyiinpirical shell or casing 2 oisuitable diele c tri'c material, such as porcelain for out:doorservice or molded material for indoor servicahaying outstanding flanges formed integrallytherwith.

T The shell" 2" is supported at its lower end on a" fianged collar or ground-band 4 with a gasket 5 interposed therebetween. The flange 4 is suitably bolted to a casing 6 which may be a tank for an oil immersed transformer or circuitbreaker.

The upper end of the casing 2 is provided with a cap 8 having a terminal in secured thereto. A conducting lead or stud [2 extends axially of the bushing and is maintained in position by insulating material M. In the present illustration, although not limited thereto, the invention contemplates an insulation M of the wound condenser type which comprises alternate layers of insulating and conducting material wound upon the conducting lead or stud i2 and of such thickness that it is maintained in position within the ground flange 4 in any of several well known ways and forms a fluid-tight joint therewith. This type of insulation is quite Well known and a further disclosure thereof is not considered necessary.

Referring to Fig. 2, it will be noted that cap 8 seats upon the top of the casing 2 with a gasket interposed therebetween and is resiliently maintained in position by means of a plurality of helical springs i6 which are maintained in compression against the base of the cap by means of a fitting l8 threaded to the upper extremity of the bushing stud l2. The degree of compression of the springs I6 is so chosen that they will always be in compression throughout the range of differential expansion between the metallic and porcelain parts of the bushing caused by the temperature variations encountered in normal service. This ensures that the gasket beneath the cap, as Well as the gasket 5 at the lower end of casing 2 are always maintained in compression to hermetically seal the bushing at these points.

The cap 8 is closed by a cover to which the terminal EU is threaded, and preferably brazed or soldered as at It. The upper end of the stud I 2 is electrically connected to the terminal [0 by means of a conductor 20 within the cap suitably clamped at its opposite ends to the terminal and stud, respectively, and being of suflicient length to permit it being so clamped before the cover is secured to the cap.

The condenser wrapping on the stud, only a portion of which is shown, comprises layers 22 of dielectric material, such as a shellaced paper, with intervening layers 24 of metallic foil. Only one layer of the metallic foil is shown, but it is to be understood that in accordance with usual construction there will be a plurality of such layers suitably spaced by dielectric layers 22 to a desired thickness.

With the construction described, a test of the bushing in accordance with usual practice would involve applying the test potential between the terminal it and the ground fiange 4. However, as above pointed out, this would also involve applying the test potential to apparatus parts within the tank 6, and this is undesirable as above indicated.

The present invention contemplates an additional layer 26 of metallic foil spaced from the stud 12 by a relatively thin layer 28 of insulating material. It is intended that the test potential shall be applied to this layer 26 and only a small amount of insulation 23 is required because there will be but little difference in potential between layer 26 and stud [2. Accordingly, the layer 28 may be substantially thinner than the layers 22, although, of course, it is within the scope of the invention to make all of the insulating layers of the same thickness if desired, or to vary the thickness to obtain a desired condenser effect.

The metal layer 26 is extended up beyond the adjacent insulating layer 22 and is exposed above the latter so that a conductor 30 within the cap may be secured thereto as by a wrapped wire 32.

The free end of the conductor 30 is then connected to an outlet, generally designated as 34, which comprises hollow cone 36 of porcelain or other insulating material, and is provided with an outstanding flange to be received in a recessed opening in the side wall of the cap 8. Suitable packing is provided around such outstanding flange and the packing is maintained in compression by a hollow plug 38 threaded to the interior of the opening in the cap wall. The portion of the porcelain cone 36 within the cap is provided at its free end with a metallic fitting 40 suitably soldered thereto and having a cylindrical portion 42 extending into the cone. The fitting 40 may be maintained in position by any of the well known expedients of soldering metal to porcelain such, for example, as disclosed in Smede et a1. Patent No. 1,852,093. The free end of the conductor 30 may be secured to the fitting, as indicated, by means of a terminal screw engaging a lug on the fitting. Also, it should be noted that preferably the portion of the cone 36 within the cap, as well as the conductor 30, should be disposed between an adjacent pair of the compression springs 16.

The outlet 34 may be closed by a plug closure 44 which has a portion threaded into the hollow plug 38 to a point where a flange on the closure 44 closes the opening through the side Wall of the cap. The plug 44 is provided with a jack 46 having a head portion for electrically engaging the cylindrical portion 42 of the fitting 40.

The foil layer 26 is insulated from the stud l2 throughout its length, but by reason of conductor 30 and the outlet 34, with its closure 44, the foil and stud are maintained at the same potential. In the normal testing operation, the test potential may be applied to the terminal 10 and the loss in the bushing as well as the connected apparatus may be determined. If the value obtained is abnormal, it is merely necessary to remove the closure 44 with its jack 46 to completely insulate the foil layer 26 from the cap as well as the stud I2. A jack similar to 46 constituting a terminal of the test lead, may then be inserted into the fitting 40 and the insulation of the bushing itself may be tested independently of the stud l2 and apparatus connected thereto.

One type of testing equipment which may be used is indicated schematically in Fig. 1, and comprises a source of alternating current energizing the low tension winding of a step-up transformer 50, the high tension winding of which is connected between ground and one corner of a bridge circuit.

The adjacent arms of the bridge include. re spectively, a variable resistor R3 and a variable condenser C4 which is in shunt with a fixed resistor R4. Another arm of the bridge contains the standard capacitor C2 and the remainin arm is connected through the test lead 52 through the bushing to be tested and hence to ground. The cross arm of the bridge contains a galvanometer 54, and by operating R3 and C4 to obtain a null reading on the galvanometer, calibrated dials on B3 and C4 can be read to give values proportional to capacitance and power factor of the bushing. Usually the test lead 52 is shielded by a sheath 56 which is preferably maintained at the test potential. In the present arrangement, th h t 56 may also be connected to the terminal in so that the stud l2 and metal foil layer 26 will be at substantially the same potential and any losses in the stud and apparatus connected thereto will be supplied from the shield circuit and Will not affect the bridge reading.

Although as shown and described the invention involves a bushing of the condenser type, but quite obviously it is applicable to a bushing of any type in which an auxiliary metallic sheath such as the metal foil 25, is disposed close to but insulated from the stud of the bushing. Quite obviously other variations in the structure shown are possible, and it is intended that the invention shall not be limited except by the appended claims.

I claim as my invention:

1. In an insulating bushing, including a stud of conducting material, a terminal electrically connected thereto for connection to an electric circuit, an insulating wrapping on said stud including a layer of conducting material insulated from said stud, and a test terminal insulated from said first mentioned terminal and means electrically connecting it to said layer of conducting material whereby a test potential may be applied to said layer to test the quality of the insulation afforded by said wrapping independently of said stud, and releasable means for bringing said stud and conducting layer to the same electrical potential for normal operation of the bushing.

2. In an insulating bushing comprising a shell of dielectric material having a metallic mounting flange thereon and a conducting stud extending therethrough for connection at one end to electrical apparatus in a grounded tank and to a metallic terminal cap for the shell at the other end, an insulating medium surrounding said stud within said shell, and means for hermetically sealing said shell and cap; a cylindrical conductor within the shell adjacent to and surrounding said stud and insulated therefrom, a test terminal accessible through an opening in a wall of said cap having an exposed portion for receiving a test lead such that a test potential may be applied directly to said cylindrical conductor to test the qualities of said insulating medium independently of losses in said stud and apparatus connected thereto, means within the bushing chamber for electrically connecting said cylindrical conductor and terminal, and means for insulating said terminal from the cap.

'3. In an insulating bushing comprising a shell of dielectric material having a metallic mounting flange thereon and a conducting stud extending therethrough for connection at one end to electrical apparatus in a grounded tank and to a metallic terminal cap for the shell at the other end, an insulating medium surrounding said stud within said shell, and means for hermetically sealing said shell and cap; a cylindrical conductor within the shell adjacent to and surrounding said stud and insulated therefrom, a test terminal accessible through an opening in a wall of said cap having an exposed portion for receiving a test lead, means within the bushing chamber for electrically connecting said cylindrical conductor and terminal, and means for insulating said terminal from the cap, including means for sealing said opening to retain the hermetically sealed condition of said shell during the application of a test potential between said terminal and said metallic mounting flange.

4. In an insulating bushing comprising a shell of dielectric material having a metallic mounting flange thereon and a conducting stud extending therethrough for connection at one end to electrical apparatus in a grounded tank and to a metallic terminal cap for the shell at the other end, an insulating medium surrounding said stud within said shell, and means for hermetically sealing said shell and cap; a cylindrical conductor within the shell adjacent to and surrounding said stud and insulated therefrom, a test terminal accessible through an opening in a wall of said cap having an exposed portion for receiving a test lead, means within the bushing chamber for electrically connecting said cylindrical conductor and terminal, means for insulating said terminal from the cap and releasable means for maintaining said stud and cylindrical conductor at the same electrical potential during normal operation of the bushing.

5. In an insulating bushing comprising a shell of dielectric material having a conducting stud extending therethrough, insulating means surrounding said stud within the shell, a tubular conducting member surrounding said stud and insulated therefrom its entire length, a flexible conductor secured at one end to said tubular conducting member, a terminal cap for closing the upper end of the shell and enclosing the adjacent end of said stud, terminal means exposed through an opening in a wall of said cap and insulated from the cap, means for securing the other end of said flexible conductor to said terminal means within the cap, and removable means for bringing said conductor and tubular conducting member to the same electrical potential for normal operation of the bushing.

6. In a bushing of the condenser type comprising a conducting stud and an insulating wrapping therefor comprising a layer of dielectric material embracing said stud, a layer of conducting material spaced from said stud by the first mentioned layer, and a plurality of additional layers of alternate dielectric and conducting material surrounding the first mentioned layers, the thickness of each of the dielectric layers of said plurality of additional layers being greater than that of the dielectric layer embracing the stud, a sealed casing for said bushing, a test terminal supported on said casing and in a position accessible from the exterior thereof, means for insulating it from said stud, and means for electrically connecting said terminal to said first-mentioned layer of conducting material.

ALEXANDER A. SKVORTZOF'F. 

